lung (n.) Look up lung at Dictionary.com
human or animal respiratory organ, c. 1300, from Old English lungen (plural), from Proto-Germanic *lungw- (source also of Old Norse lunge, Old Frisian lungen, Middle Dutch longhe, Dutch long, Old High German lungun, German lunge "lung"), literally "the light organ," from PIE *legwh- "not heavy, having little weight; easy, agile, nimble" (source also of Russian lëgkij, Polish lekki "light;" Russian lëgkoje "lung," Greek elaphros "light" in weight; see also lever).

So called perhaps because in a cook pot lungs of a slaughtered animal float, while the heart, liver, etc., do not. Compare Portuguese leve "lung," from Latin levis "light;" Irish scaman "lungs," from scaman "light;" Welsh ysgyfaint "lungs," from ysgafn "light." See also lights, pulmonary. Lung cancer is attested from 1882. Lung-power "strength of voice" is from 1900.